Flash memory array with dual function control lines and asymmetrical source and drain junctions

ABSTRACT

A flash memory formed by a process wherein at least two parallel stacked gate strips are formed on a silicon substrate such that the stacked gate strips are separated by field oxide islands. Asymmetrical first and second junctions are formed in each of a set of source/drain regions and a chemical etch is applied to form the field oxide islands into oxide spacers that align a dual-function control line to the first and second junctions. The resulting flash memory includes a plurality of stacked gate islands, one or more source/drain regions between at least a subset of the plurality of stacked gate islands, first junctions in each of the source/drain regions, second junctions in each of the source/drain regions and dual function control lines in the source/drain regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional Application of, and claims the benefitof the filing date of the following Application: FLASH MEMORY ARRAY WITHDUAL FUNCTION CONTROL LINES AND ASYMMETRICAL SOURCE AND DRAIN JUNCTIONS,by Michael A. Van Buskirk and Chi Chang, filed on Jan. 16, 1998, U.S.Ser. No. 09/008,162, now U.S. Pat. No. 6,492,675; which is incorporatedherein by reference.

This Application is related to the following Application: PROCESS FORFABRICATING A FLASH MEMORY ARRAY WITH DUAL FUNCTION CONTROL LINES, byMichael A. Van Buskirk and Chi Chang, filed on Jan. 16, 1998, Ser. No.09/008,415, issued as U.S. Pat. No. 6,001,689; which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention pertains to the field of non-volatile memories.More particularly, this invention relates to a non-volatile memory arraythat includes self-aligned dual function bit lines and asymmetricalsource and drain junctions.

2. Description of the Related Art

Prior non-volatile memories typically include an array of memory cellswhich are accessible via a matrix of control lines. Such control linestypically include dedicated bit lines that couple to the drain regionsof the memory cells. In addition, such control lines usually includeword lines that couple to the control gates of the memory cells anddedicated V_(ss) lines that couple to the source regions of the memorycells.

Such dedicated bit lines are typically formed from a metal such asaluminum. Such word lines are commonly formed of a polysilicon materialor polycide material that includes a refractory metal. Such prior memoryarrays typically include drain contact areas that provide electricalcoupling between the drain regions of the memory cells and the dedicatedbit lines.

Such drain contact areas are usually defined by the photo lithographicprinting equipment employed in the particular memory devicemanufacturing process. Typically, such printing equipment ischaracterized by a minimum feature size for forming areas on the memoryarray including the drain contact areas. As a consequence, each draincontact area consumes a fixed minimum area of integrated circuit diespace according to the minimum feature size inherent with the particularprinting equipment.

Unfortunately, the integrated circuit die space consumed by such draincontact features limits the density of a memory array for a given areaof integrated circuit die space. Such density limitations commonlyrequire that high capacity memory arrays be implemented on largersilicon substrates. Such larger silicon substrates usually increases theoverall cost of such memory devices.

One prior method for eliminating such drain contact features in anonvolatile memory array is to employ an array architecture that may bereferred to as a virtual ground architecture. Such a virtual groundarray includes dual-function control lines that function as both bitlines and V_(ss) lines. Such a virtual ground array usually obviates theneed for forming drain contact features for dedicated bit lines.

The dual function control lines in prior virtual ground memory arraysare typically formed by masking, dopant implant, and thermal diffusionprocess steps. Such a process that employs masking and thermal diffusionprocess steps usually causes migration of the implanted dopant intoareas outside of the control line areas defined by the mask. Such dopantmigration typically requires increased spacing of control lines in orderto provide electrical isolation of the control lines from other areas ofthe memory array. Unfortunately, such increased spacing of control linesusually reduces the density of memory cells formed by such a process.

SUMMARY

The present invention, roughly described, provides for a flash memoryformed by a process wherein at least two parallel stacked gate stripsare formed on a silicon substrate such that the stacked gate strips areseparated by a plurality of field oxide islands. A first junction and asecond junction are then formed in each of a set of source/drain regionslocated between the stacked gate strips and between the field oxideislands. A chemical etch is then applied to form the field oxide islandsinto pairs of oxide spacers and to expose a continuous strip of thesilicon substrate between the oxide spacers and over the source/drainregions. A dopant is implanted into the continuous strip to form acontrol line to the first and second junctions such that the controlline is aligned by the oxide spacers.

One embodiment of the flash memory device includes a plurality ofstacked gate islands, one or more source/drain regions between at leasta subset of the plurality of stacked gate islands, a first junction ineach of the source/drain regions, a second junction in each of thesource/drain regions and a dual function control line in thesource/drain regions. The first junction and second junction areasymmetrical. Another embodiment of the flash memory device includes astacked gate structure, a first source/drain region, a first junction inthe first source/drain region, a second junction in the firstsource/drain region, a first dual function control line in the firstsource/drain region, a second source/drain region, a third junction inthe second source/drain region, a fourth junction in the secondsource/drain region and a second dual function control line in thesecond source/drain region.

A flash memory of the present invention can be operated by one of manyvarious alternative methods. One embodiment method includes programminga flash memory cell using a first source/drain region as a drain and asecond source/drain region as a source; and reading the flash memorycell using the first source/drain region as the source and the secondsource/drain region as the drain. In another embodiment, the methodincludes connecting the second dual function control to a groundpotential while programming the flash memory, and connecting the firstdual function control line to the ground potential while reading theflash memory cell.

In yet another embodiment, the method of operating the flash memory cellincludes programming the flash memory cell using the first source/drainregion as a drain and the second source/drain region as a source suchthat the step of programming includes transferring electrons from thefloating gate to the drain.

The flash memory is read using the second source/drain region as a drainand the first source/drain region as a source.

Other objects, features and advantages of the present invention will beapparent from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with respect to particular exemplaryembodiments thereof and reference is accordingly made to the drawings.

FIG. 1 is a top view of a core area of a flash memory after a series offront end processing steps that form a set of field oxide islands and aset of stacked gate strips.

FIG. 2A provides a cross-sectional view of the flash memory alongsection AA.

FIG. 2B provides a cross-sectional view of the flash memory alongsection BB.

FIG. 3 is a top view of the core area which shows doped regions forsource/drain functions formed in the silicon substrate between the fieldoxide islands.

FIG. 4 is a view along the cross-section DD which illustrates theformation of the n⁺ doped regions.

FIG. 5 is a view along the cross-section DD which illustrates theformation of the n doped regions.

FIG. 6A is a view along cross-section CC of the core area after thefirst chemical etching step on the spacer oxide layer.

FIG. 6B shows a view along cross-section DD of the core area after thefirst chemical etching step on the spacer oxide layer.

FIG. 7A provides a view along cross-section CC after formation of thecontinuous n⁺ strips for the dual-function control lines.

FIG. 7B provides a view along cross-section DD after formation of thecontinuous n⁺ strips for the dual-function control lines.

FIG. 8 is a top view of a core area which shows a set of continuous n⁺strips of dual function control lines formed between the etched oxidespacers.

FIG. 9A provides a view along cross-section EE after the chemical etchthat planarizes the surface of the core area.

FIG. 9B provides a view along cross-section FF after the chemical etchthat planarizes the surface of the core area.

FIG. 10 is a top view of the core area which shows the localinterconnect word lines.

FIG. 11 is a top view of the core area including cutaway portionswhich-reveal the remaining active regions of the stacked gate strips andhidden asymmetrical source/drain junctions.

FIG. 12A provides a view along cross-section GG of a set of memory cellsin the core area.

FIG. 12B provides a view along cross-section HH of a set of memory cellsin the core area.

FIG. 12C provides a view along cross-section II of a set of memory cellsin the core area

FIG. 13 provides a cross-sectional view of memory cells implementing analternative program/erase/read scheme.

DETAILED DESCRIPTION

FIG. 1 is a top view of a core area 10 of a flash memory after a seriesof front end processing steps. The front end processing steps on thecore area 10 form a set of field oxide islands, a subset of which arereferenced as field oxide islands 20-34. The field oxide islands 20-34delineate isolation areas and active areas for flash memory cells in thecore area 10. The front end processing steps also form a set of stackedgate strips 230-234 in the core area 10.

FIGS. 2A and 2B provide cross-sectional views along sections AA and BB,respectively. FIG. 2A shows the field oxide islands 26-29 deposited ontoa silicon substrate 200 that contains the core area 10. A set of thinoxide layers are deposited on the silicon substrate 200 between thefield oxide islands 20-34 including a set of thin oxide layers 61-63deposited on the silicon substrate 200 between the field oxide islands26-27, 27-28, and 28-29.

Each stacked gate strip 230-234 includes a pair of polysilicon layersseparated by a dielectric region. For example, the stacked gate strip231 includes a polysilicon layer 41 and a polysilicon layer 51 separatedby a dielectric region 71. The polysilicon layer 41 is formed into acontrol gate for a flash memory cell in core area 10. Polysilicon layer51 is formed into a floating gate for the same flash memory cell in corearea 10. Similarly, the polysilicon layers 42 and 52 and the polysiliconlayers 43 and 53 provide two more flash memory cells in the core area10.

The stacked gate strips 230-234 are initially formed by a deposition ofa first sheet of polysilicon onto the field oxide islands and the thinoxide layers in the core area 10. The first sheet of polysiliconprovides material for the floating gates 51-53. In one embodiment, thefirst sheet of polysilicon is deposited according to a chemical vapordeposition process step. Alternatively, the first sheet of polysiliconmay be substituted with a layer of amorphous silicon.

In one embodiment, the dielectric regions 71-73 include a set ofoxide-nitride-oxide layers which are deposited onto the first sheet ofpolysilicon. An initial oxide layer is formed by a chemical vapordeposition process step. Thereafter a layer of nitride is deposited ontothe initial oxide layer. Finally, a top oxide layer is grown ordeposited onto the nitride layer to form an oxide-nitride-oxidestructure for the dielectric regions 71-73.

The formation of the stacked gate strips 230-234 continues with adeposition of a second sheet of polysilicon onto the oxide-nitride-oxidelayers. The second sheet of polysilicon provides material for thecontrol gates 41-43. Alternatively, the second sheet of polysilicon maybe substituted with a layer of amorphous silicon.

The layers in the core area 10 that include the first and second sheetsof polysilicon separated by the oxide-nitride-oxide dielectric regionare then masked and etched into long strips to form the stacked gatestrips 230-234.

FIG. 3 is a top view of the core area 10 which shows doped regionsformed in the silicon substrate 200 between the field oxide islands20-34. The doped regions includes strips of n⁺ doped regions andoverlapping strips of n⁻ doped regions formed between adjacent pairs offield oxide islands 20-34 in a direction parallel to the stacked gatestrips 230-234. For example, a set of n⁺ doped regions 80-83 are formedbetween field oxide island pairs 26 and 31, 27 and 32, 28 and 33, and 29and 34, respectively. A set of n⁻ doped regions 90-93 are formedadjacent to and overlapping the n⁺ doped regions 80-83. The n⁺ and n⁻doped regions provide asymmetrical source/drain junctions.

FIG. 4 is a view of the core area 10 along the cross-section DD whichillustrates the formation of the n³⁰ doped regions 81 and 82. Initially,the n⁺ doped regions 81 and 82 are defined by a masking step applied tothe core area 10. The masking step defines long strips located betweenthe stacked gate strips 230-234 and delineate the n⁺ doped regionsincluding n⁺ doped regions 81 and 82. After the masking step, an n⁺ typedopant is implanted into the silicon substrate 200 into the unmaskedregions including the regions 81 and 82. The implant mask is thenremoved to expose the entire area between the stacked gate strips230-234 including the n⁺ doped regions 81-82.

FIG. 5 is a view of the core area 10 along the cross-section DD whichillustrates the formation of the n⁻ doped regions 91 and 92. The removalof n+ mask exposes the areas of the silicon substrate 200 between thestacked gate strips 230-234. An n⁻ dopant is implanted into the areas 91and 92 as well as the areas 81 and 82.

After formation of the n⁺ and n⁻ doped regions between the stacked gatestrips 230-234, a spacer oxide layer 210 is deposited over the core area10. In one embodiment the spacer oxide layer is formed by a chemicalvapor deposition step. The spacer oxide layer 210 then undergoes a firstchemical etching step.

FIG. 6A is a view along cross-section CC of the core area 10 after thefirst chemical etching step on the spacer oxide layer 210. The firstchemical etch of the spacer oxide layer forms spacer regions between thestacked gate strips 230-234. For example, a gap 101 is formed betweenthe stacked gate strips 231 and 232 and a gap 102 is formed between thestacked gate strips 232 and 233. As shown, the first chemical etch ofthe spacer oxide layer causes partial removal of the field oxide islandsas shown by the field oxide islands 27 and 28.

FIG. 6B shows a view along cross-section DD of the core area 10 afterthe first chemical etching step on the spacer oxide layer 210. The firstchemical etch causes removal of part of spacer oxide layer 210 down to asurface 212 of the silicon substrate 200. The first chemical etch ishighly selective toward removing oxide rather than silicon.

Thereafter a second chemical etch is applied to the core area 10 whichremoves the remainder of the field oxide islands 20-34 not covered byspacer oxide layer 210, down to the surface 212 of the silicon substrate200. The first and second oxide etching steps applied to the spaceroxide layer 210 are highly preferential to etch oxide and not silicon.

A set of continuous n⁺ strips are then formed by implantation of an n⁺dopant into the silicon substrate 200. The continuous n⁺ strips arealigned by the oxide spacers between the stacked gate strips 230-234.Each continuous n⁺ strip provides a dual function control line thatfunctions as both a bit line and a V_(ss) line for flash memory cells inthe core area 10.

FIGS. 7A and 7B provide views along cross-sections CC and DD,respectively, after formation of the continuous n⁺ strips including then⁺ strips 111 and 112. The first and second oxide spacer chemical etchesremove the spacer oxide layer 210 from surfaces of the stacked gatestrips 230-234 and down to the silicon substrate 200 in between thestacked gate strips 230-234. The remnants of the spacer oxide layer 210and the field oxide island 27 yields oxide spacers 121 and 122. Theoxide spacers 121 and 122 control the alignment of the n⁺ strip 111. Theoxide spacers 121 and 122 insure proper electrical isolation between then⁺ strip 111 and an adjacent flash memory cells formed from the stackedgate strips 231 and 232. FIG. 7B shows n⁺ strip 111 in relation toregions 81 and 91.

FIG. 8 is a top view of a core area 10 which shows a set of continuousn⁺ strips 110-113 formed between the oxide spacers. The continuous n⁺strips 110-113 provide electrical paths to the asymmetrical source anddrain junctions of the flash memory cells of the core area 10. The n⁺strips function as bit and V_(ss) lines. The n⁺ strips that function asbit lines during read operations on the flash memory cells in the corearea 10 also function as V_(ss) lines during programming operations onthe flash memory cells in the core area 10. The n⁺ strips that functionas V_(ss) lines during read operations on the flash memory cells in thecore area 10 also function as bit lines during programming operations onthe flash memory cells in the core area 10.

The core area 10 is then planarized prior to word line formation.Initially, an oxide layer is deposited over the core area 10 with achemical vapor deposition process step. The oxide layer undergoes anetch back step which planarizes the core area 10 and facilitatesformation of word lines for the flash memory cells. FIGS. 9A and 9Bprovide views along cross-sections EE and FF, respectively, after thechemical etch that planarizes the surface of the core area 10. Theplanarization oxide layer fills in the oxide spacers 101 and 102 and thesubsequent chemical etch exposes the surfaces 131-133 of the stackedgate strips 231-233.

A salicidation process is then performed on the core area 10 to bridgethe stacked gate strips 230-234 and provide word line interconnect. Theword line interconnect in one embodiment includes a titanium silicidelayer and a layer of amorphous silicon deposited onto the surfaces131-133. The word line interconnect is then masked and chemically etchedto form local interconnect word lines in the core area 10. The localinterconnect wordlines are perpendicular to the direction of theoriginal stacked gate strips 230-234. The etching step also removesremaining areas of the stacked gate strips 230-234 outside the word lineinterconnect areas.

FIG. 10 is a top view of the core area 10 which shows the localinterconnect word lines 140-144. The portions of the stacked gate strips230-234 outside the word lines 140-144 are etched away to revealremaining portions of field oxide islands 20-34 along with the dualfunction continuous n⁺ strips which are aligned by the oxide spacerprovided by the remnants of the field oxide islands 20-34.

FIG. 11 is a top view of the core area 10 including cutaway portionswhich reveal the remaining active areas (or islands) of the stacked gatestrips 230-234 as well as hidden doped regions for source and drainjunctions.

FIG. 12A provides a view along cross-section GG of the core area. Theremaining oxide regions including the oxide spacers 120-124 along withthe planarization oxide in the spacers 101 and 102 provide electricalisolation for the continuous n⁺ strips 111 and 112.

FIGS. 12B and 12C are views along cross-sections HH and II of the corearea, respectively. The cross-sectional views show a titanium layer 162and an amorphous silicon layer 160 deposited thereon. The titanium layer162 and the amorphous silicon layer 160 together form a word lineinterconnect among the control gates 40-43. Approximately 350 Angstromsthick of titanium is either ‘sputtered’ or ‘CVD-deposited’ onto thewafer surface, followed by an approximately 750 Angstroms thick ofamorphous silicon deposition by sputtering. Then, heat treatment isapplied (typically, by using RTA, or rapid thermal annealing, process)to react the titanium and amorphous silicon to form titanium silicide.An alternative method of forming this local interconnect includes usinga low-temperature CVD WSix (tungsten silicide) deposition of a thicknessof about 1500 Angstroms. There are other more ‘exotic’ methods, such asusing tungsten (W) metal scheme, that can also achieve the same purpose.

An integrated circuit using the flash memory core described above willalso have periphery transistors that are used to implement other logicand functions. These periphery transistors will not have stacked gates.The periphery transistors will get the “silicided” gates (a titaniumlayer and an amorphous silicon layer) like the core array cells. Unlikemainstream microprocessor technology, these periphery transistors'source and drain silicon surfaces will not be “salicided” with thisscheme. As a result, these periphery transistors' n+ or p+junctions canwithstand relatively high voltages, are less leaky and have higherjunction breakdown voltages as compared to the salicided junctions. Ifthe Ti/amorphous silicon (to form titanium silicide local interconnectand gates) scheme is used, it is much easier to fabricate the p+dopedpolysilicon gate p-channel transistors, without as much concern of boronimpurity penetration into gate oxide/transistor channel as otherwisewould if the WSix gate is used and with a reaction gas using WF6. Amajor disadvantage of p+ doped gate p-channel transistor is better“off-state” or “turn-off” characteristics.

The basic functions of the flash memory cells in the core area 10 aredescribed using an example flash memory cell that includes a controlgate from the polysilicon layer 42 and a floating gate from thepolysilicon layer 52. The n⁻ doped region 91 and the n⁺ doped region 81provide asymmetrical source and drain junctions for the flash memorycell. The n⁻ doped region 91 is a source junction during programming andis a drain junction during reading. The n⁺ doped regions 81 is a drainjunction during programming and is a source junction during reading.

During program operations using source side injection, the dual functionn⁺ strip 111 provides a source path to the flash memory cell while thedual function n⁺ strip 112 provides a drain path to the same flashmemory cell. The n⁻ doped region 91 yields a relatively high resistivepath on the source side of the flash memory cell during programming. Theresistivity of the n⁻ doped region 91 heats or energizes electrons thattravel between the source and the drain through the channel of the flashmemory cell. Some electrons attain sufficient energy near the n⁻ dopedregion 91 to travel across the oxide layer 62 to the floating gate 52.

This phenomenon of electron injection to the floating gate 52 near thesource side of a flash memory cell is referred to as source sideinjection. Source side injection reduces power consumption duringprogramming because a higher rate of electron injection to the floatinggate occurs in comparison to the drain side injection whichcharacterizes flash memory cells having symmetrical source and drainjunctions.

To achieve the source side injection during programming the source pathis connected to ground, the drain path is connected to medium positivevoltage and the control gate is connected to a high positive voltage. Amedium positive voltage can be between approximately three andapproximately eight volts. A high positive voltage can be betweenapproximately six and approximately twelve volts. Looking at FIG. 12,for example, strip 111 can be connected to ground (or ground potential),strip 112 can be connected to five volts, and gate 42 can be connectedto positive eight volts for program operation.

For erase operations, strip 111 is connected to a low positive voltage.The low positive voltage can be between a voltage slightly above zeroand three volts. Strip 112 is connected to a medium positive voltage orVcc. Gate 42 is connected a negative high voltage.

During read operations, n⁺ strip 111 functions as a drain path while then⁺ strip 112 functions as a source path. This reversal of the source andthe drain junctions prevents the high resistivity of the n⁻ doped region91 from hindering the available levels of read current in the flashmemory cell. To perform a read, strip 111 is connected to a low positivevoltage, strip 112 is connected to ground and gate 42 is connected toVcc.

Instead of using the source side injection as described above, thepresent invention flash memory can be operated using a DINOR process. Adescription of the DINOR (Divided bit line NOR) structure and processcan be found in “A Novel Cell Structure Suitable For A 3 Volt Operation,Sector Erase Flash Memory,” H. Onoda, Y. Kunori, S. Kobayashi, M. Ohi,A. Fukumoto, N. Ajika and H. Miyoshi, LSI Laboratory, MitsubishiElectric Corporation, 1992, IEEE 600-IEDM 92; and “Memory ArrayArchitecture and Decoding Scheme for 3 V Only Sector Erasable DINORFlash Memory” Shin-Ichi Kobayashi, Hiroaki Nakai, Yuichi Kunori, TakeshiNakayama, Yoshikazu Miyawaki, Yasushi Terada, Hiroshi Onada, NatsuoAjika, Masahiro Hatanaka, Hirokazu Miyoshi, and Tsutomu Yoshihara, 1994IEEE, Journal of Solid State Circuits Vol. 29, No. 4, April 1994; bothincorporated by reference. This alternative process will be describedwith respect to FIG. 13. Note that the stacked gate islands, junctionsand dual control lines of FIG. 13 are the same as described with respectto FIG. 12C. However, FIG. 13 further depicts a p-well 202 inside an-well 204.

During a program operation using a DINOR process, strip 111 acting as asource path, is connected to a low positive voltage (e.g., 1 volt).Strip 112, acting as a drain path, is connected to a medium positivevoltage (e.g., 5 volts). Additionally, gate 41 is connected to a highnegative voltage (between −7 and −10 volts), p-well 202 is grounded andn-well 204 is grounded. In the DINOR process, electrons from floatinggate 52 are transferred to the drain. Note that during a programmingoperation, a neighboring flash memory cell can be inhibited so that itwill not be programmed inadvertently. That is, the n⁻ junction carries alarge voltage drop from the applied bit line such that the electricfield across the tunnel oxide at the neighbor memory cell is smallerthan the intended cell and the electrons stored on the floating gate ofthe neighbor cell will not be discharged. Note that in FIG. 13, thememory cell with gate 43 is a neighbor to the memory cell with gate 42.

During an erase operation, strip 111 is connected to ground of leftfloating, strip 112 is connected to ground of left floating, controlgate 42 is connected to a high positive voltage (e.g., between 7 and 10volts), p-well 202 is connected to a negative high voltage preferablybetween negative eight to negative ten volts and n-well 204 is grounded.

During read operations, strip 111 functions as a drain while strip 112functions as a source. Strip 111 is connected to a low voltage, strip112 is grounded, gate 41 is connected to Vcc, p-well 202 is grounded andn-well 204 is grounded.

The foregoing detailed description of the invention has been presentedfor purpose of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The described embodiment was chosen in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. A method of operating a flash memory cell havinga control gate, the flash memory cell having a first dual functioncontrol line in a first source/drain region and a second dual functioncontrol line in a second source/drain region, comprising the steps of:programming the flash memory cell using the first source/drain region asa drain and the second source/drain region as a source; and reading theflash memory cell using the second source/drain region as a drain andthe first source/drain region as a source; wherein the step ofprogramming includes providing a voltage having a magnitude of less thanten volts to the control gate.
 2. A method according to claim 1, whereinthe step of programming includes: providing a high positive voltage tothe control gate; providing a medium positive voltage to the drain; andproviding a ground potential to the source.
 3. A method according toclaim 2, wherein the high positive voltage is approximately eight voltsand the medium positive voltage is approximately five volts.
 4. A methodaccording to claim 1, wherein the step of programming includes:providing a high negative voltage to the control gate; providing amedium positive voltage to the drain; and providing a low voltage to thesource.
 5. A method according to claim 1, wherein the step of readingincludes: providing a medium positive voltage to the control gate;providing a low positive voltage to the drain; and providing a groundpotential to the source.
 6. A method according to claim 5, wherein thelow positive voltage is approximately one volt and the medium positivevoltage is approximately five volts.
 7. A method according to claim 1,wherein the second dual function control line is connected to a groundpotential during the step of programming and the first dual functioncontrol line is connected to a ground potential during the step ofreading.
 8. A method of operating a flash memory cell having a controlgate, the flash memory cell having a first dual function control line ina first source/drain region and a second dual function control line in asecond source/drain region, comprising the steps of: connecting thesecond dual function control line to a ground potential and programmingthe flash memory cell; and connecting the first dual function controlline to the ground potential and reading the flash memory cell; whereinthe step of programming includes providing a voltage having a magnitudeof less than ten volts to the control gate.
 9. A method according toclaim 8, wherein the step of programming includes: providing a highpositive voltage to the control gate; providing a medium positivevoltage to the drain; and providing a ground potential to the source.10. The method according to claim 8, wherein the step of readingincludes: providing a medium positive voltage to the control gate;providing a low positive voltage to the drain; and providing a groundpotential to the source.
 11. A method of operating a flash memory cell,the flash memory cell having a control gate, a floating gate, a channel,a first dual function control line in a first source/drain region, asecond dual function control line in a second source/drain region, an-well and a p-well inside the n-well, comprising the steps of:programming the flash memory cell using the first source/drain region asa drain and the second source/drain region as a source, the step ofprogramming includes transferring electrons from the floating gate tothe drain; and reading the flash memory cell using the secondsource/drain region as a drain and the first source/drain region as asource; wherein the step of programming includes providing a voltagehaving a magnitude of less than ten volts to the control gate.
 12. Themethod according to claim 11, wherein: the step of programming the flashmemory cell is performed without programming a neighboring flash memorycell.
 13. A method according to claim 11, further comprising the stepof: erasing the flash memory cell, including providing a high positivevoltage to the control gate, providing a medium positive voltage to thedrain, and providing a ground potential to the source.
 14. A methodaccording to claim 11, wherein the step of programming includes:providing a high negative voltage to the control gate; providing amedium positive voltage to the drain; and providing a low voltage to thesource.
 15. A method according to claim 11, wherein the second dualfunction control line is connected to a ground potential during the stepof erasing and the first dual function control line is connected to aground potential during the step of reading.